The present invention relates to a water spraying system as defined in the preamble of claim 1.
The invention concerns specifically a system for supplying an aqueous liquid mist into the intake air of a preferably turbocharged piston engine to reduce nitrogen oxide emissions (NOx). At the high combustion temperatures, the combustion process in the cylinders of a piston engine produces nitrogen oxides, which are emitted together with the exhaust gas into the atmosphere. Because of the harmful climatic effects of nitrogen oxide emissions, efforts are undertaken to minimize their production.
As is known, adding water to the combustion process either in the form of water vapor or water droplets reduces the formation of nitrogen oxides. This phenomenon is based on a cooling effect. When the water sprayed into the cylinder is evaporated, it reduces the temperature of the air in the cylinder while at the same time reducing the pressure. The pressure drop has an adverse effect on the efficiency, although the decrease of pressure and temperature has a favorable effect on the formation of nitrogen oxides. When the water is supplied in the form of droplets together with the intake air, some of it is additionally wasted during the scavenging period and water consumption is increased. When air saturated with water vapor is supplied into the cylinder, the thermal capacity of the filling gas is increased and the gas has a substantially greater effect of reducing the temperatures of the combustion process than does dry air. The effect of reducing the combustion temperatures increases with the water vapor concentration, yet without producing an undesirable effect on efficiency. Since an increase in the temperature of the gas supplied into the cylinder also augments the generation of nitrogen oxides as well as the consumption of water, it is desirable to keep the gas temperature as low as possible, yet high enough to ensure that the gas supplied into the cylinder contains an amount of water vapor sufficient for the reduction of nitrogen oxides.
An apparatus for vaporizing a desired amount of water is disclosed in U.S. Pat. Nos. 5,758,606 and 6,196,165. A drawback with this apparatus is that the device mounted between the turbocharger and the cylinder increases the cubic volume of the air intake ductwork, which has a considerable effect on the power output of the engine. The power output is dependent on the cubic volume after the turbocharger because during power increase or decrease the air pressure produced by the turbocharger increases the density of the air and the amount of gas supplied into the cylinder. If the cubic volume between the turbocharger and the cylinder is increased, then it will take considerably longer before the amount of air produced by the turbocharger brings the pressure to the desired level and the power generated by the engine increases. Another drawback with the apparatus is that the heated water used for vaporization and flushed over the evaporation surfaces has the effect of increasing the temperature of the air. The device is unable to make use of the cooling effect produced in connection with the vaporization of the water, but the gas output from the device is at a relatively high temperature, so the amount of water vapor required for the reduction of nitrogen oxides and therefore also the water consumption are increased considerably.
Specification WO98/10185 again discloses an apparatus In which the air produced by a turbocharger and the pressure of this air are utilized in the injection of water for humidifying the air supplied to the turbocharger. A drawback with this system is the relatively low temperature of the supply air, which is why the amount of water vapor evaporated into the air remains small, and thus no significant nitrogen oxide reducing effect is achieved. Another drawback is that when the amount of water is increased, the water droplets can not be evaporated after the air has reached a saturated state, with the result that the water droplets drift into the turbocharger and cause wear of the turbocharger vanes through droplet erosion. From a thermodynamical viewpoint, the drifting of droplets into the turbocharger is desirable as it reduces the work performed by the turbocharger, increasing the pressure of the pressurized air produced at the output and simultaneously reducing its temperature. In practice, however, an air compressor rotating at a very high speed—about 50,000–100,000 rpm—has proved to be very sensitive to droplet erosion as referred to above.
The object of the present invention is to achieve a water spraying system designed for supplying water mist into the air intake ductwork of especially a piston engine and allowing the drawbacks of prior-art to be avoided.
The system of the invention is mainly characterized in that the system comprises means for producing adjustable water mist spraying at at least one point in the air intake duct, depending on the load and/or speed of rotation and/or temperature of the engine.
The system of the invention is additionally characterized by what is stated in claims 2–17.
The solution of the invention has numerous significant advantages. In the apparatus of the invention, the above-described undesirable effects and deficiencies are eliminated by using adjustable water spraying, which is distributed to one or more points in the air intake duct by varying the number and/or size and/or quality of the nozzles used, depending on the load and temperature of the engine. According to the invention, the water flux is distributed to a number of small nozzles to make it possible to produce sufficiently small droplets and/or to distribute such droplets over a larger are in the air intake duct so as to achieve an optimal vaporization. In the system of the invention, the spraying can also be focused on optimal points in the air intake ductwork where the temperature and/or air flow is highest. In the system of the invention, the number of nozzles spraying, the point and/or direction of injection of the spray in the air intake ductwork can be varied according to the amount of water needed, e.g. on the basis of the load and/or speed of rotation of the engine. In the system of the invention, it is further possible to maintain a high nozzle pressure so as to keep the droplet size of the mist being sprayed sufficiently small. Furthermore, the system allows the spraying to be varied between nozzles having different properties. The system of the invention produces an optimal droplet size of the liquid injected into the intake air. By using a nozzle cleaning system as part of the water spraying system, very reliable operation of the system is achieved because the possibility of nozzles being clogged is avoided. By using pop-up nozzles as part of the system, the risk of the nozzles being clogged is further reduced. On the other hand, by using pop-up nozzles, the nozzles are no impediment to the flow in the air intake duct when the system is out of use.
As the apparatus is connected directly to the structures of the air intake duct and it produces a fine mist directly without using any extra chambers or other containers, it is able to make full use of the heat quantity required for the vaporization of the water, cooling the intake air at each spray injection point to a temperature close to the wet bulb temperature (or adiabatic saturation temperature, which in the case of a water-air mixture is practically the same thing), i.e. to the temperature to which the air temperature can be reduced by vaporization of water. As connecting the apparatus of the invention to a turbocharged engine does not involve any changes in the cubic volume of the air intake system, it has no adverse effect on the power output of the engine, either.
Another advantage of the invention is that the humidity of the intake air can be increased stepwise after each heat supply point, thus adjusting the humidity of the gas fed into the cylinder and therefore the formation of nitrogen oxides within desired limits.